This invention relates to speed limiters for internal combustion engines, and more particularly to speed limiters for small internal combustion engines of the type used to power lawn mowers, snow blowers, generators and the like.
It is often desirable to limit the speed of an engine to a predetermined maximum or limit speed. Many types of electronic speed limiters are known. One type operates off the engine's alternator. Since the alternator typically provides a voltage proportional to the engine speed, controlling the maximum voltage that may be reached by the alternator then controls the engine's maximum speed.
Another type of engine speed limiter compares a signal functionally related to the engine speed with a reference signal functionally related to a maximum limit speed. If the result of the comparison indicates that the actual engine speed is higher than the maximum limit speed, ignition pulses from the primary winding are grounded so that the engine coasts down to a lower speed. Although the ignition pulses are being grounded, fuel is still being pumped into the combustion chamber. A great deal of fuel collects in the combustion chamber and is exhausted through the exhaust system. The exhausted fuel may be ignited by the hot muffler, causing backfire. This backfire results in an undesirable loud noise and may harm the engine components.
Other types of electronic speed limiters generate a reference speed signal that is not easily changed. For example, U.S. Pat. No. 5,009,208 issued Apr. 23, 1991 to Fiorenza, II and assigned to Briggs & Stratton Corporation discloses an electronic speed limiter using discrete frequency dividers and other components to compute the reference signal from a one MHz input signal. Although Fiorenza '208 discloses a very accurate engine speed limiter which only grounds a preselected number of ignition pulses, neither the reference speed signal nor the preselected number of grounded ignition pulses may be changed.
U.S. Pat. No. 3,767,972 issued Oct. 23, 1973 to Noddings et al discloses an analog system in which a reference voltage may be stored in an electrical memory circuit, and may be changed by the operator. However, the Noddings et al system is very complicated and expensive since it requires many components parts. It is also very difficult to accurately adjust the reference voltage due to the nature of the analog devices being used.